Ovarian Tissue Cryopreservation for Fertility Preservation

7.1

Overview

The survival rate among the ~2% of women of reproductive age who have suffered from invasive cancer has substantially increased. Unfortunately, the necessary chemo- and radiotherapy carry the risk of unwanted side effects such as permanent infertility, jeopardizing a woman’s chances of having her own biological children.

Until recently, in vitro fertilization (IVF) and embryo transfer (ET) in combination with cryopreservation were considered the only options for women to conceive after recovery from a sterilizing cancer treatment. However, these methods carry some drawbacks and cannot sustain long-term fertility. They require controlled ovarian stimulation, and are not applicable to prepubertal girls. Cryopreservation and transplantation of ovarian tissue fulfils a number of these shortcomings. Grafting of cryopreserved ovaria.;n tissue can restore menstrual cyclicity, does not require pretreatment and can be performed without delay. Further, the method does not require male gametes and is applicable even to prepubertal girls.

It is very difficult to give a patient an estimation of the risk of premature ovarian insufficiency (POI) due to the number of contributory factors, including age, disease, stage of disease and the fact that the planned chemotherapy treatment often changes during the course of treatment. To help the physicians evaluate each patient, selection criteria such as the Edinburgh criteria ( Figure 7.1 ) can be used for guidance. However, these criteria should merely be used as guidance and not exact guidelines, as each woman should have an individual medical assessment of her ovarian reserve as well as her risk of POI. At the same time, she should have the opportunity to decide whether she is willing to take the risk of the operation in relation to the risk of becoming infertile. In Denmark, there is no exact upper age limit for women having ovarian tissue cryopreserved because some women have a better fertility potential than others (e.g., a high number of antral follicles) despite increased biological age (>35 years of age) and would therefore still benefit from having ovarian tissue cryopreserved. Furthermore, some women are still interested in having ovarian tissue stored even though their own risk of POI is fairly low, as they simply do not want to risk being unable to have children. Other women wish to have their ovary cryopreserved, even when their estimated chances of survival are vague, because the chance of survival is present. When the woman herself needs to cover the cost of cryopreservation, for example as this service is offered in USA, the situation becomes even more difficult. It is therefore of utmost importance to give these women information about their specific situation: their estimated chances of surviving the cancer and furthermore an estimation of their fertility potential, and to inform them that the current experience is rather limited and it is not known to what extent women of, for instance, advanced reproductive age may actually benefit from having tissue transplanted. A parallel situation is seen in IVF treatment, where women of advanced reproductive age receive treatment despite the fact that the success rate is rather low.

7.2

Ovarian Tissue Cryopreservation

The success of ovarian cryopreservation is based on the high cryopreservation tolerance of small primordial follicles compared to the larger growing follicles. The vast majority of primordial follicles are located in the outermost 1 or 2 mm of the ovarian cortex, which is relatively easy to isolate from the rest of the ovarian tissue (see Figure 7.1 ). The frozen ovarian cortex is stored in liquid nitrogen, allowing time for the patient to recover after cancer treatment is completed.

The most widely used protocol for ovarian cryopreservation is the slow-freezing method, and up until now all live births in humans, except for one birth from Japan, have been achieved after slow-freezing. The most used media compositions include either dimethyl sulfoxide (DMSO) or ethylene glycol, both in combination with sucrose. However, vitrification of ovarian tissue could be one way of improving outcomes after freezing and re-implantation, as new results from non-human primates have shown superior follicular viability and development of antral follicles. Despite these encouraging results, the slow-freezing method is still preferred over vitrification.

Currently there is no consensus as to how much of the ovarian cortex should be harvested for cryopreservation. It is recommended that oophorectomy should be performed on patients receiving high doses of alkylating agents, patients undergoing pelvic irradiation or total body irradiation, and younger prepubertal girls due to the small size of their ovaries. For other patients, 4–5 ovarian cortical biopsies are harvested in some countries, whereas a unilateral oophorectomy is carried out systematically in other countries. The advantages of taking a whole ovary include minimizing the possibility of post-operational complications, e.g., bleeding, and also increasing the possibility of either grafting a large pool of follicles, which potentially should provide fertility with a higher efficacy, or performing repeated transplantation in case the tissue of the first transplantation becomes exhausted. However, the decision of excising one whole ovary in contrast to biopsies remains an issue of debate.

Transportation of ovarian tissue prior to cryopreservation allows hospitals without cryopreservation expertise to treat women locally for the cancer disease and just send the ovarian tissue to the centre that performs cryopreservation. This facilitates quality control, proper equipment, and personnel to fulfil clinical, legal and scientific standards required for proper conduction of the procedure. The feasibility of centralized cryobanking has been proven in Denmark and Germany, where ovarian tissue has been transported up to 20 hours prior to freezing, and these principles are now being introduced in many other countries worldwide. The applicability of this approach will, however, be determined by local regulations.